Field of the Invention
The present disclosure relates to an image forming apparatus which forms an image with an electrostatic latent image.
Description of the Related Art
In an image forming apparatus which forms an image with an electrostatic latent image, there is a problem that the image size of the image formed on the transferring member differs from that of the image originally intended. This is due to the fact that the size of the image is influenced by the variation in the attachment position and/or heat generated at the time of fixing the formed image.
For such a problem, US2008/089585 (A1) discloses an image forming apparatus which compares an image size of the output image with that of an originally intended image; and measures the magnification in the formation process of the image based on the comparison result. Then, the apparatus performs a correction for the image data of the input image at the measured magnification, and forms an image, based on the corrected image data after the correction. As a result, an image having originally intended size is finally output.
For example, in some image forming apparatuses, when an image formation is performed, an image size of an output image may be 101[%] of that of an input image in a sub-scanning direction (i.e., a rotating direction of a photoreceptor). In this case, in order to output an image having the same image size as an input image, the image data is corrected so that the image formation is performed in the image size obtained by multiplying the original image size by the reciprocal of the measured magnification. Specifically, an image having an image size multiplied by 0.990099 (i.e., reciprocal of 101[%]=100/101[%]) in the sub-scanning direction is formed. As a result, the image size of the output image becomes 1.010101 . . . *99[%]=100[%] in a main scanning direction, and becomes 0.990099*101[%]=100[%] in the sub-scanning direction.
In the magnification changing method of the input image data in the image processing device described in US2008/089585 (A1), the number of pixels, which corresponds to the difference of the image size of the output image being output without correction and the desired image size in the sub-scanning direction, is calculated. Then, based on the number of pixels, an image area is divided, in the sub-scanning direction, into a plurality of areas. FIG. 15 is a diagram schematically illustrating the input image to be processed. The input image illustrated in FIG. 15 is a set of pixels arranged in the main scanning direction and in the sub-scanning direction. For example, suppose that the difference of the desired image size and the size of the output image output without correction is 10 pixels. In this case, as illustrated in the dotted line in FIG. 15, an input image is divided into 10 areas along the sub-scanning direction. Further, for each divided area, based on a random number value generated by a random number generator, one random position is determined for every coordinate position (main scanning position) in a main scanning direction. Even in the same area, the random positions may differ depending on the sub-scanning position. At the determined random position, a pixel is inserted when enlarging the image size in the sub-scanning direction, and a pixel is deleted when reducing the image size. When a pixel has been inserted, then a pixel positioned at one-line behind the determined random position in the sub-scanning direction is output. This is called “delayed output”. When a pixel has been deleted, then a pixel positioned at one-line advanced of the determined random position in the sub-scanning direction is output. This is called “advanced output”. These “delayed” and “advanced” states, which are generated by the insertion or the deletion of a pixel, will be initialized upon changing of the area.
FIG. 16 is, for a certain area, a schematic diagram for visualizing and representing the random position determined based on the random number value generated by a random number generator. As illustrated in FIG. 16, the coordinate value representing the main scanning position of the area is “x”, and the coordinate value representing a line number (sub-scanning position) is “y.” In FIG. 16, the dotted lines which are parallel to the main scanning direction are boundary lines (area boundaries) of areas, and are equivalent to the dotted lines illustrated in FIG. 15. For example, when enlarging the image size of an input image in the sub-scanning direction, a pixel is inserted at a determined random position. Because of inserting a pixel, an image as illustrated in FIG. 17A (the image before enlarging) is, as illustrated in FIG. 17B, output with its image size expanded by a correction process (enlarging process). This correction process is a magnification changing process in which the image size is enlarged by one line in the sub-scanning direction in one area. When reducing the image size in the sub-scanning direction, a pixel is deleted in the determined random position. As a result of deleting a pixel, the image as illustrated in FIG. 17C (the image before reduction) is output with its image size being reduced by the correction process (reducing process), as illustrated in FIG. 17D. This correction process is a magnification changing process in which the image size is reduced by one line in the sub-scanning direction in one area.
Some image forming apparatuses includes a frame buffer for storing image data. In this case, the magnification changing process in the sub-scanning direction is performed by performing a correction process for the input image data stored in the frame buffer. In addition, a band process is also employable as another method for storing image data. By employing the band process, the amount of a line memory (line buffer) in the image forming apparatus is minimized. Therefore, an expensive frame buffer is not necessary for the image forming apparatus, and reduction of the manufacture cost of the image forming apparatus can be achieved.
Here, details of the band process will be described. FIG. 18 is a diagram schematically illustrating the input image. In the input image illustrated in FIG. 18, 14 pixels are arranged in the main scanning direction, 20 pixels are arranged in the sub-scanning direction, and one square represents one pixel. The character illustrated in each square is provided for distinguishing, for convenience, the position in the sub-scanning direction for each pixel before performing the correction process. As to a band process, it is a process for reading, from the image data representing the whole input image, a predetermined number of lines (i.e., band processing width) in the image data, and performing image processing on the read image data. In this process, reading out of the image data and image processing for the read data are repeatedly performed until the process for the whole input image is completed. As an example, in the input image illustrated in FIG. 18, if the number of lines read by one scan is four lines, as illustrated in FIG. 19, the image data consisting of 4 lines is output for every scan (i.e., scan 1 to scan 5 in FIG. 19). Whole input image (FIG. 18) will be completed by repeating 4-line reading total of 5 times, since there are 20 lines in the sub-scanning direction.
For example, when performing a magnification changing in the sub-scanning direction in a band process which reads 4 lines at a time, generally, the number of a line buffer required for this case is 5. For each area and for each main scanning position, the number of a random position for inserting or deleting a pixel is one. Therefore, when enlarging the image size, what is necessary is to prepare, for the random position, one additional line which is just behind the present line. Further, when reducing the image size, what is necessary is to prepare, for the random position, one additional line which is adjacent and advancing the present line. As an example, description is given where a magnification changing process is performed on the image data output of scan 2 illustrated in FIG. 19.
FIG. 20 is a figure for explaining the number of line buffers required for a magnification changing process (the number of lines). When a pixel is inserted in the line at the sub-scanning position A as illustrated in FIG. 20, for example, the line data of the sub-scanning position X which is the last line in the scan 1 (FIG. 19) is needed. When deleting a pixel in the line of sub-scanning position D, the line data of sub-scanning position Y which is a line of the first line in the scan 3 (FIG. 19) is needed. Since the magnification changing process is one of an enlarging process or reducing process, the required number of line buffers is 5 (i.e., the sum of 4 and 1).
However, when the band process is performed in the scan including the area boundary of each divided area, the number of line buffers may exceed “band process width+1” and two random number generators may be needed. Details of this case will be specifically described as below. FIG. 21 illustrates a portion of an input image having 9 pixels for the length (sub-scanning area length) of the sub-scanning direction of an area. The dotted lines in FIG. 21 illustrates the boundary lines of areas, and an area boundary is defined between pixel X (character in a square is “X”) and pixels 1 (character in a square is “1”), between pixel 9 (character in a square is 9) and pixels 1, and between pixel 9 and pixel Y (character in a square is “Y”). In FIG. 21, seen from a front side, the upper area is defined as “area A” and downward area is defined as “area B”. Hereinafter, description is given where the magnification changing process for reducing the image size is performed, with 4 lines band width, on the area A and the area B.
FIG. 22 illustrates, in the area A and the area B illustrated in FIG. 21, a state in which random positions, at which a pixel is deleted, are specified. FIG. 23 illustrates the result of deleting pixels at the positions illustrated in FIG. 22, i.e., the result of performing the magnification changing process for reducing the image size. The positions of the shaded square illustrated in FIG. 23 represent the positions at which a pixel is deleted in the magnification changing process for reduction. In FIG. 23, each of the areas A and the area B is reduced by one line in the sub-scanning direction in the magnification changing process, as a result, the sub-scanning area length is reduced from 9 pixels to 8 pixels.
FIG. 24 illustrates: 1) an output image data (from scan 1 to scan 5) when the band process is performed on the image illustrated in FIG. 23 with band width being 4 lines; and 2) line data and the number of the line buffers required for the magnification changing process. Following description is given in a case where the magnification of the sub-scanning direction of an image forming apparatus is small enough and the cycle of the insertion or the deletion of a pixel is longer than the band process width. Specifically, in a case where the band process width is four lines, in the magnification changing rate, the cycle of the insertion or the deletion of a pixel is more than or equal to four lines. As to the magnification in which frequency of insertion of a pixel is one per 4 lines, since image data consisting of 4 lines is changed into image data consisting of 5 lines, the magnification is 5/4=1.25. Therefore, the magnification changing rate is 125[%]. Further, as to the magnification in which the deletion of a pixel occurs once per 4 lines, since image data consisting of lines is changed into image data consisting of 3 lines, the magnification is ¾=0.75. Therefore, the magnification changing rate is 75[%]. In the following, description is given assuming that the magnification changing rate is in the range of 75[%] to 125[%]. It is noted that the range of the magnification changing rate frequently used in a known image forming apparatus is 99[%] to 101[%]. The above-mentioned magnification range may differ in the image forming apparatus with a different band process width. However, even in the case, the band process width is about 32 lines at the maximum, therefore, as compared to the range of 99[%] to 101[%], the above mentioned range of 75[%] to 125[%] is sufficiently wide.
As illustrated in FIG. 24, in scan 1, scan 2, scan 4, and scan 5, the required number of the line buffers is five. However, in scan 3, as illustrated, the required number of line buffers is six. Among scans 1 to 5, only in scan 3, the required number of line buffers increases one. This is due to the fact that scan 3 is performed over the area A and the area B for performing the band process on these lines. Usually, for each main scanning position, the insertion or the deletion of a pixel occurs once for every scan.
However, in a scan over two or more areas, Depending on the combination of 1) the random number value used for the process of the area A; and 2) the random number value used for the process of the area B, in the same main scanning position, the insertion or the deletion of a pixel may occur twice. For example, in the main scanning position P of scan 3 illustrated in FIG. 24, the deletion of a pixel has occurred twice. When performing the band process on the line over the area A and area B, a random number value should be generated for each of the area A and the area B. Further, the generated random number values must be stored. Therefore, both line buffers for image processing for a scan over the areas and two random number generators should be previously provided in an image forming apparatus. Therefore, there remains a problem that the manufacturing cost of an image forming apparatus is increased.
The present disclosure is directed to provide an image forming apparatus which can reduce the required number of line buffers and random number generation circuits when performing a magnification changing process in a band process.